Recent estimates support the notion that a relatively small fraction of the
information available on the Web is accessible through search engines. Furthermore,
many search engines return irrelevant results.

Databases typically disclose their contents only in response to a query. For
databases accessible through the Web this means that their contents are ephemeral
parts of the Web only being disclosed through a query. This makes it virtually
impossible for search engines to index their contents.

While search engines are useful, there are a number of other types of resources
that need to be considered when using the Internet for research.

These make up what some have called the invisible Web or the hidden
Internet. The primary resources of the invisible Web are called specialized
databases. Virtual libraries, meta-search tools that include lists of databases,
and directories are often the best sources to use when looking for specialized
databases.

http://www.scout.cs.wisc.edu/report/sr/current/index.html
The Scout Report is a good way to keep up with new search tools, especially
specialized databases. You can view its weekly report and its archive of
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to scout-report-request@cs.wisc.edu. Type subscribe to scout-report
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